The present invention relates to apparatus and methods for microscopic inspection of articles and more particularly to an apparatus and methods for detecting defects in a VLSI reticle by comparison to a reference.
Apparatus and methods for automatically inspecting photomasks or reticles used in manufacturing VLSI circuits have been described in the literature and have been commercially available for a number of years.
These systems generally employ an optical sensor to provide an electronic representation of the inspected object so that a comparison to another similar object or electronic representation can be made. In order to detect non-random, i.e. repeating, defects the comparison is generally made against a known good reference. If the reference is the design database from which the inspected article was manufactured, a generally complete set of defect types may be detected. An inspection system may be characterized by the different types of articles it can inspect, the type of reference it uses, the types and sizes of defects it detects, and the time it requires to inspect a complete article. Modern VLSI technology typically requires the detection of 0.5 micron defects on a single die 5xc3x97reticle in 10 minutes.
Inspection apparatus is disclosed in U.S. Pat. Nos. 4,247,203 and 4,347,001, both to Levy et al. The apparatus described in those patents locates faults in the photomask by simultaneously comparing adjacent dies on the photomask and locating differences. Because a known good die is not used in this type of inspection apparatus, only random, i.e. non-repeating, faults are generally identifiable, and not repeating faults.
U.S. Pat. No. 4,579,455, also to Levy et al, describes attempts to improve the detection efficiency of inspection apparatus especially at photomask pattern edges. An area subtraction technique is used to identify defects by detecting differences between substantially duplicate die patterns in a photomask. Two square window matrices of seven rows and seven columns of adjacent pixels are defined for corresponding areas of two die patterns in a single photomask. The center 3xc3x973 matrix of each window matrix is defined as a comparison matrix with each window matrix having twenty five unique subsets of 3xc3x973 adjacent pixels within its boundaries; one in the center and twenty four others that are offset by one or two pixels from the center in one or both directions.
An error value is then calculated for each subset of each matrix by summarizing the squares of the differences between each of the nine pixel values of each subset and the corresponding pixel value of the opposite comparison matrix. If there is no defect, and misalignment between the two representations is less than two pixels, at least one error value will be less than a threshold error value. If none of the twenty five error values relating to one comparison matrix are less than the threshold value, a defect is assumed to be located within the comparison matrix or within the opposite window matrix. The magnitude of the threshold error is then automatically varied according to the number of edges within the window matrices to compensate for edge induced errors.
U.S. Pat. No. 4,532,650 to Wihl et al describes attempts to improve the defect efficiency of inspection apparatus especially near photomask pattern corners. The detection process is based upon using a vector gradient within a matrix to develop candidate and canceller information for each representation. This information is then logically manipulated to qualify the data obtained and to determine whether or not a defect has been detected.
U.S. Pat. No. 4,805,123 to Specht et al describes a photomask and reticle inspection system in which a first stream of data representing the image content of a selected surface area of an object to be inspected is compared to a second stream of data representing the intended image content of the selected surface area of the object. After misalignment between stored portions of the two streams of data is detected and corrected, using shifts of an integral number of pixels and/or subpixel interpolation methods, subportions are compared in order to detect differences therebetween in excess of a predetermined threshold.
U.S. Pat. No. 4,926,489 to Danielson et al. describes a die-to-database reticle inspection system including timing control means which is responsive to misalignment between the inspected object and the database. The timing control means is used to generate control signals which adjust the relative output signal rates of the scanner and the database thereby maintaining the misalignment below a predetermined limit.
The present invention seeks to provide an improved system for inspection of and detection of defects in objects such as reticles, photomasks, semiconductor wafers, flat panel displays and other patterned objects. Preferably, the system has two or more stages, whereby the object is examined separately for fine defects, preferably by inspecting a binary level representation of the object, and for ultra fine defects, preferably by inspecting a gray level representation of the object.
The two-stage inspection system has the advantage of allowing different algorithms to be used for fine defect detection and for ultra fine defect detection, each algorithm being generally optimal for defects of the size it is intended for but not necessarily for defects of other sizes. The system may make die to database comparisons or die to die comparisons.
The system also preferably includes reinspection apparatus for reinspection of detected defects, thereby to reduce the false alarm rate, and for classifying the remaining defects by size, area and type. Preferably the system also includes diagnostic apparatus for performing diagnostic functions such as inspecting the flow of information through the system and diagnosing which component of the system may be misoperating.
There is thus provided in accordance with a preferred embodiment of the present invention an inspection method including the steps of providing a patterned object to be inspected and compared with a reference, inspecting the patterned object and providing an output of information relating to the visual characteristics of the patterned object, comparing binary level information relating to the visual characteristics of the patterned object to binary level information relating to the visual characteristics of the reference, and comparing gray level information relating to the visual characteristics of the patterned object to gray level information relating to the visual characteristics of the reference.
There is also provided in accordance with a further preferred embodiment of the present invention an inspection method including the steps of providing a patterned object to be inspected and compared with a reference, inspecting the patterned object and providing an output of information relating to the visual characteristics of the patterned object, and providing an output indication of differences between the patterned object and the reference, the step of providing including the steps of sensing hill profiles and valley profiles in information relating to the visual characteristics of the patterned object, sensing hill profiles and valley profiles in information relating to the visual characteristics of the reference and providing a defect indication when a hill profile or a valley profile of at least a sufficient level is sensed for a given location in the patterned object and not for a corresponding location in the reference.
There is also provided in accordance with a preferred embodiment of the present invention an inspection method for providing an output indication of differences between an patterned object and a reference and including the steps of providing a patterned object to be inspected and compared with a reference, comparing binary level information relating to the visual characteristics of the patterned object to binary level information relating to the visual characteristics of the reference and filtering out differences within a selected magnitude range.
There is further provided in accordance with yet a further preferred embodiment of the present invention an inspection method including the steps of providing a patterned object to be inspected and compared with a reference, initially inspecting the patterned object and providing an output of information relating to visual characteristics of the patterned object, providing an output indication of differences between the patterned object and the reference representing possible defects, and simultaneously storing information regarding visual characteristics of areas of the patterned object at which possible defects are indicated.
Further in accordance with a preferred embodiment of the present invention, the method also includes the step of automatically carrying out a further inspection of the areas of the patterned object using the stored information.
Still further in accordance with a preferred embodiment of the present invention, the step of automatically carrying out includes the step of comparing at least some of the areas to a reference.
Additionally in accordance with a preferred embodiment of the present invention, the method also includes the step of displaying the stored information to an operator.
In accordance with still a further preferred embodiment of the present invention there is provided an inspection method including the steps of providing a patterned object to be inspected and compared with a reference, inspecting the patterned object and providing a representation of visual characteristics of the patterned object, providing an output indication of differences between the patterned object and a representation of visual characteristics of the reference, at least one of the inspecting step and the difference indicating step including the step of providing at least one subsystem which modifies at least one of the representations, thereby to provide a plurality of representations of at least one of the patterned object and the reference, and inspecting the flow of information through the inspecting apparatus and the difference indicating apparatus and including comparing individual ones from among the plurality of representations.
There is also provided in accordance with a further preferred embodiment of the present invention an inspection method including the steps of providing a patterned object to be inspected and compared with a reference, inspecting the patterned object and providing an output of information relating to the visual characteristics of the patterned object, providing an indication of differences between the patterned object and the reference, and automatically carrying out a second inspection of at least some areas of the patterned object thereby to complement the providing step by enhancing the output indication of differences provided by the providing step.
There is further provided in accordance with still a further preferred embodiment of the present invention an inspection system including apparatus for comparing an inspected object to a reference and providing an output of information relating to the visual characteristics of the inspected object, binary level comparison apparatus for comparing binary level information relating to the visual characteristics of the inspected object to binary level information relating to the visual characteristics of the reference, and gray level comparison apparatus for comparing gray level information relating to the visual characteristics of the inspected object to gray level information relating to the visual characteristics of the reference.
Further in accordance with a preferred embodiment of the present invention, the system also includes apparatus for providing information relating to the visual characteristics of the reference.
Still further in accordance with a preferred embodiment of the present invention, the gray level comparison apparatus includes apparatus for converting binary level information relating to visual characteristics of the reference into gray level information.
Additionally in accordance with a preferred embodiment of the present invention, the binary level comparison apparatus is operative to sense differences of at least a first minimum size and the gray level comparison apparatus is operative to sense differences of at least a second minimum size smaller than the first minimum size.
Still further in accordance with a preferred embodiment of the present invention, the gray level comparison apparatus includes first apparatus for sensing hill profiles and valley profiles in the information relating to the visual characteristics of the inspected object, second apparatus for sensing hill profiles and valley profiles in the information relating to the visual characteristics of the reference, and apparatus for providing a defect indication when a hill profile or a valley profile of at least a sufficient level is sensed for a given location in the inspected object and not for a corresponding location in the reference.
Further in accordance with a preferred embodiment of the present invention, the system also includes apparatus responsive to selected information indicating the presence of selected features at locations in the reference for preventing provision of an output indication of difference in respect of those locations.
Still further in accordance with a preferred embodiment of the present invention, the location in an individual one of the reference and the inspected object is larger in area than the corresponding location in the other one of the reference and the inspected object.
Additionally in accordance with a preferred embodiment of the present invention, the second apparatus for sensing is operative with a higher sensitivity than the sensitivity of the first apparatus, thereby to reduce the false alarm rate of the apparatus for providing a defect indication.
There is also provided in accordance with yet a further preferred embodiment of the present invention an inspection system for providing an output indication of differences between an inspected object and a reference and including binary level comparison apparatus for comparing binary level information relating to the visual characteristics of the inspected object to binary level information relating to the visual characteristics of the reference, and apparatus for providing an output indication of differences and including apparatus for filtering out differences within a selected magnitude range.
Further in accordance with a preferred embodiment of the present invention, the binary comparison apparatus includes at least one of the following: at least one SUBTRACTION logical operator, and at least one XOR logical operator.
Still further in accordance with a preferred embodiment of the present invention, the apparatus for filtering includes an EROSION operator and/or an ACCEPTANCE operator.
Additionally in accordance with a preferred embodiment of the present invention, the apparatus for filtering includes apparatus for differentiating between differences between the inspected object and the reference on the basis of the location of each difference relative to at least one edge of an adjacent binary shape defined in at least one of the inspected object and the reference.
There is also provided in accordance with yet a further preferred embodiment of the present invention an inspection system including apparatus for initially inspecting an inspected object and providing an output of information relating to visual characteristics of the inspected object and apparatus for providing an output indication of differences between the inspected object and a reference, representing possible defects, and for simultaneously storing information regarding visual characteristics of areas of the inspected object at which possible defects are indicated.
Further in accordance with a preferred embodiment of the present invention, the system also includes further inspection apparatus for automatically carrying out a further inspection of the areas of the inspected object using the stored information.
Still further in accordance with a preferred embodiment of the present invention, the apparatus for automatically carrying out includes apparatus for comparing at least some of the areas to a reference.
Additionally in accordance with a preferred embodiment of the present invention, the system also includes apparatus for displaying the stored information to an operator.
Still further in accordance with a preferred embodiment of the present invention, the system also includes apparatus for providing information relating to the visual characteristics of the reference.
Further in accordance with a preferred embodiment of the present invention, the apparatus for storing is operative to store the information provided by the apparatus for initially inspecting.
Still further in accordance with a preferred embodiment of the present invention, the apparatus for storing is operative to store an output indication of differences received from the apparatus for providing an output indication of differences.
Additionally in accordance with a preferred embodiment of the present invention, the further inspection apparatus includes apparatus for classifying at least some of the possible defects in accordance with at least one criterion.
Still further in accordance with a preferred embodiment of the present invention, the apparatus for carrying out a second inspection is characterized in that the second inspection of areas of an inspected object may be initiated while the apparatus for providing an output indication is operating.
Further in accordance with a preferred embodiment of the present invention, the apparatus for carrying out a second inspection is characterized in that the second inspection of areas of an inspected object may be initiated while the apparatus for providing an output indication is operating on the inspected object.
There is also provided in accordance with a further preferred embodiment of the present invention an inspection system including inspecting apparatus for inspecting an inspected object and providing a representation of visual characteristics of the inspected object, difference indicating apparatus for providing an output indication of differences between the inspected object and a representation of visual characteristics of a reference, at least one of the inspecting apparatus and the difference indicating apparatus defining at least one subsystem which modifies at least one of the representations, thereby to provide a plurality of representations of at least one of the inspected object and the reference, and information flow inspection apparatus for inspecting the flow of information through the inspecting apparatus and the difference indicating apparatus and including apparatus for comparing individual ones from among a plurality of representations of an object.
Further in accordance with a preferred embodiment of the present invention, the information flow inspection apparatus includes apparatus for providing input directly to a selected one of the at least one subsystems.
Still further in accordance with a preferred embodiment of the present invention, the input includes a representation of visual characteristics of one of the inspected object and reference outputs by a subsystem upstream of the selected subsystem.
Further in accordance with a preferred embodiment of the present invention, the input includes a test pattern.
Still further in accordance with a preferred embodiment of the present invention, the information flow inspection apparatus includes apparatus for inspecting the output of a selected one of the at least one subsystems.
Additionally in accordance with a preferred embodiment of the present invention, the system includes apparatus for providing a representation of information relating to visual characteristics of a selected object to a selected one of the at least one subsystems, thereby to provide a plurality of representations of information related to visual characteristics of the selected object.
There is also provided in accordance with still a further preferred embodiment of the present invention an inspection system including apparatus for comparing an inspected object to a reference and providing an output of information relating to the visual characteristics of the inspected object, first apparatus for providing an indication of differences between the inspected object and the reference, and second apparatus for automatically carrying out a second inspection of at least some areas of the inspected object and for complementing the first apparatus and including apparatus for enhancing the output indication of differences provided by the first apparatus.
Further in accordance with a preferred embodiment of the present invention, the apparatus for enhancing includes apparatus for preventing provision of the output indication of difference in at least some areas of the inspected object, thereby to reduce the false alarm rate of the first apparatus.
Still further in accordance with a preferred embodiment of the present invention, the second apparatus includes apparatus for providing an output indication of differences between the inspected object and the reference not detected by the first apparatus.
There is further provided in accordance with still another preferred embodiment of the present invention alignment apparatus including apparatus for providing binary level information relating to a patterned object to be inspected, thereby to define an inspected object channel, apparatus for providing binary level information relating to a reference, thereby to define a reference channel, and apparatus for aligning the inspected object channel and the reference channel using the binary level information.
Further in accordance with a preferred embodiment of the present invention, the apparatus for aligning includes first apparatus for comparing a portion of the binary level information relating to the inspected object to a corresponding portion of the binary level information relating to the reference, thereby to provide an output indication of misalignment, and second apparatus for receiving the output indication of misalignment from the first apparatus and for aligning the corresponding portions of the binary level information relating to the inspected object and the reference.
Further in accordance with a preferred embodiment of the present invention, the system also includes apparatus for receiving information relating to the visual characteristics of the reference from a database precursor of the reference.
Still further in accordance with a preferred embodiment of the present invention, the apparatus for inspecting includes apparatus for inspecting the reference and providing information relating to the visual characteristics of the reference.
Additionally in accordance with a preferred embodiment of the present invention, the system also includes apparatus for inspecting the reference and providing information relating to the visual characteristics of the reference.
Still further in accordance with a preferred embodiment of the present invention, the apparatus for inspecting includes apparatus for varying the magnification at which at least the inspected object is inspected.
Further in accordance with a preferred embodiment of the present invention, the apparatus for inspecting also includes apparatus for automatically controlling, as a function of the magnification, parameters relating to the inspection of at least the inspected object.
Further in accordance with a preferred embodiment of the present invention, the patterned object includes an individual one of the following group: a flat panel display, a reticle, a printed circuit board, a semiconductor wafer, a photomask, and a blank of any of the above.
In accordance with still a further preferred embodiment of the present invention, there is provided an inspection system including apparatus for providing at least one representation of the visual characteristics of an inspected object, first apparatus for comparing an individual one of the at least one representations of the inspected object to a reference and for providing an output indication of differences, and second apparatus for comparing an individual one of the at least one representations to a reference, thereby to complement the first apparatus by enhancing the indication of differences provided by the first apparatus.
According to still a further preferred embodiment of the present invention, there is provided diagnostics apparatus for diagnosing the operation of an information processing system, the information processing system including a plurality of subsystems, each individual one from among the plurality of subsystems being operative to receive a first representation of information to be processed and to output a second representation of the information to be processed, the apparatus including information flow diagnosing apparatus for comparing individual ones from among the plurality of representations.
Still further in accordance with a preferred embodiment of the present invention, the information flow diagnosing apparatus is synchronized with the information processing system.
Additionally in accordance with a preferred embodiment of the present invention, the information flow diagnosing apparatus is electronically integrated with the information processing system.
Further in accordance with a preferred embodiment of the present invention, the information flow diagnosing apparatus includes apparatus for comparing individual ones from among the plurality of representations in real time.
Further in accordance with a preferred embodiment of the present invention, the apparatus also includes apparatus for providing input, such as a test pattern, directly to a selected one of the at least one subsystems.
Still further in accordance with a preferred embodiment of the present invention, the apparatus also includes apparatus for receiving the output of a selected one of the at least one subsystems.
Additionally in accordance with a preferred embodiment of the present invention, the apparatus for receiving includes apparatus for recording the output of the selected one of the least one subsystems and apparatus for off-line processing of the recorded output.